The use of rechargeable cells and batteries has been steadily and rapidly increasing in recent years. With developments in rechargeable cell technology, rechargeable cells are used for power not only in traditional areas such as flashlights and small electronic devices but also in heavy duty power tools, temporary power sources, small vehicles such as forklifts, and even larger vehicles such as small automobiles. Of course, rechargeable cells are also widely used in the starting systems of gasoline powered vehicles and tools. Strictly speaking, a "battery" is a set of cells in series or parallel. For purposes of the present invention, however, the terms may be used interchangeably.
A characteristic of rechargeable cells is that the internal resistance of the cells and the electrochemical reaction in the cells during recharging tends to produce heat. This heat, in turn, further increases the internal resistance, which leads to still more heat. There is thus a cycle of heat production during recharging which if left uncontrolled will damage or even explode the cell.
In some nickel-cadmium cell rechargers, excessive heat build-up is prevented by use of a heat sensitive switch such as a bimetallic switch. If the temperature of the switch exceeds a predetermined switch activation temperature, the switch is opened to reduce the recharging current from full current to a trickle charge current or to cut-off the recharging current altogether. The thermocouple switch approach is relatively imprecise, since the temperature of the switch will be affected somewhat by the proximity of the switch to the cells being recharged and even by the ambient air temperature. In the case of nickel-cadmium cells, however, these variables are not very important since nickel-cadmium cells will withstand fairly high temperatures without being damaged.
There have recently been dramatic advances in lead-acid cells, as demonstrated by the thin-plate lead-acid batteries disclosed in U.S. Pat. No. 4,047,300 by Juergens and assigned to the assignee of the present invention. Lead-acid cells are less resistant to damage from high temperatures than are nickel-cadmium cells. For example, placing a typical rechargeable lead-acid cell into the type of nickel-cadmium recharger that has a heat sensitive cutoff-switch would cause high-temperature damage to the cell even before the temperature reached a sufficient level to open the switch to convert from full current to a maintenance charge current or no current. One approach to avoid this is to use a heat sensitive switch with a lower switch-off threshold, so that the switch will open at a lower temperature before the cell is heat damaged. Such an approach is not very feasible, however, because the lower temperature would need to be so low that the switch would be affected by the temperature variables mentioned above such as the precise placement of the switch in relation to the cell and ambient air temperature. A practical cutoff circuit is described in U.S. patent application No. 08/079,056 by Keating and assigned to the assignee of the present invention, where charging is reduced from full current to a maintenance charge current after a predetermined voltage of the charging cell is exceeded.
Another approach to cell charging is to use a timer to limit the duration of recharging. The timer may be used to either abruptly terminate the charging after a certain time has elapsed, or to continuously reduce the charging current as time elapses. However, batteries may develop different charging characteristics as they are successively discharged and recharged. Therefore, a cell charger using a constant charging time will not provide optimal charging. Other rechargers look for an upper voltage threshold to terminate charging. This still does not provide optimal charging because cells will obtain different maximum voltages at different states in their life-cycles. In the field of nickel-cadmium batteries, U.S. Pat. No.5,196,780 teaches a recharger in which the charging is discontinued when the cell voltage begins to decline on overcharging as is common for these types of cells.